Weld residual stress effects on fatigue crack growth behaviour of aluminium alloy 2024-T351

The interaction between residual stress and fatigue crack growth rate has been investigated in middle tension and compact tension specimens machined from a variable polarity plasma arc welded aluminium alloy 2024-T351 plate. The specimens were tested at three levels of applied constant stress intensity factor range. Crack closure was continuously monitored using an eddy current transducer and the residual stresses were measured with neutron diffraction. The effect of the residual stresses on the fatigue crack behaviour was modelled for both specimen geometries using two approaches: a crack closure approach where the effective stress intensity factor was computed; and a residual stress approach where the effect of the residual stresses on the stress ratio was considered. Good correlation between the experimental results and the predictions were found for the effective stress intensity factor approach at a high stress intensity factor range whereas the residual stress approach yielded good predictions at low and moderate stress intensity factor ranges. In particular, the residual stresses accelerated the fatigue crack growth rate in the middle tension specimen whereas they decelerated the growth rate in the compact tension sample, demonstrating the importance of accurately evaluating the residual stresses in welded specimens which will be used to produce damage tolerance design data.     

An experimental study of the critical stress intensity factor in laminated structures at high loading rates

A method for the determination of the critical stress intensity factor at high loading rates is introduced. Reflexions of stress waves, which disturb the measurement when conventional types of specimens are used, can be avoided almost entirely for a specimen made in the form of a ring. A small notch constitutes the only deviation from full circular symmetry. The ring is subjected to an interior pressure, obtained through the action of electromagnetic forces, arising from a current induced in the ring itself. As an illustration of the use of the ring-shaped specimen, an experiment is performed for determining the fracture toughness and the energy absorption before crack growth of laminated material. The ring was laminated in the radial direction. It is shown that this lamination improves the apparent fracture toughness of the ring and increases its energy absorption, in comparison with a ring of homogeneous material.     

A method for determining the stress intensity factor of a single edge-notched tensile specimen

A straightforward method for determining the stress intensity factor of a single edge-notched tensile specimen with unknown end conditions is presented. It involves the use of measured crack mouth opening displacements, coupled with existing solutions for stress intensity factors and crack opening displacements for uniform tensile loading and for pure bending. The use of the method is demonstrated through experiments on a porous matrix ceramic composite.     

Limited weld residual stress measurements in fatigue crack propagation: Part II. FEM-based fatigue crack propagation with complete residual stress fields

Using a limited set of residual stress measurements acquired by neutron diffraction and an equilibrium‐based, weighted least square algorithm to reconstruct the complete residual stress tensor field from the measured residual stress data, the effect of weld residual stress on fatigue crack propagation is investigated for 2024‐T351 aluminium alloy plate joined by friction stir welding. Through incorporation of the least squares, complete equilibrated residual stress field into a finite element model of the Friction Stir Weld (FSW) region, progressive crack growth along a direction perpendicular to the welding line is simulated as part of the analysis. Both the residual stress redistribution and the stress intensity factor due to the residual stress field, K_res, are calculated during the crack extension process. Results show that (a) incorporation of the complete, self‐equilibrated residual stress field into a finite element (FE) model of the specimen provides a robust, hybrid approach for assessing the importance of residual stress on fatigue crack propagation, (b) the calculated stress‐intensity factor due to the residual stress field, K_res, has the same trend as measured experimentally by the ‘cut‐compliance method’ and (c) the da/dN results are readily explained with reference to the effect of the residual stress field on the applied stress intensity factor.     

Determination of stress intensity factors and J-integrals using the method of caustics

Applications of the optical shadow method of reflective caustics to the measurement of the stress intensity factor and J-integral in various specimens are investigated. The necessary experimental requirements to help in determining an accurate stress intensity factor and J-integral are described. The ratios of r₀ (radius of initial curve)/r_p, (plastic zone size) and r₀/t (thickness of specimen) are found to be very important experimental parameters with which to obtain meaningful stress and/or strain intensities surrounding crack tips. The appropriate ranges to determine accurate values of stress intensity factor and J-integral for polycarbonate (compact tension) and aluminum (c-shaped tension) specimens are presented.     

Effect of residual stress on delamination from interface edge between nano-films

The focus in this study is on the effect of residual stress on the delamination crack initiation from the interface edge between thin films, Cu/TiN, where the stress is intensified by the free edge effect. The delamination tests, where the mechanical stress is applied on the interface, show that the specimen with the thinner Cu film has an apparently higher strength at the interface edge. The residual stress in the films is then evaluated by curvature measurement of film/substrate coupon and the influence on the delamination is analyzed. The residual stress increases with the increase of film thickness and remarkably intensifies the stress near the edge. By superimposing the contributions of the applied load and the residual stress, a good agreement is obtained in the normal stress intensity near the interface edge at the delamination independent of the Cu thickness. This signifies that the combination of intensified stresses due to the applied load and the residual stress governs the crack initiation at the interface edge, and the toughness at the interface edge is evaluated by the stress intensity factor on the basis of the fracture mechanics concept.     

TC4钛合金的超高周疲劳行为及其竞争失效模型构建

针对现有模型对TC4竞争失效预测的不准确性,建立了基于最大应力强度因子的竞争失效模型。在室温以及两种应力比下,针对TC4钛合金进行超高周疲劳试验,通过试验与最弱键竞争失效理论相结合的方法进行评估,研究其超高周疲劳性能。通过对试样断口形貌的观察,可将其失效模式分为如下两类:表面失效以及内部失效。对试样表面缺陷以及内部解理刻面尺寸进行测量,并评估其最大应力强度因子值。进一步通过正态分布得到最大应力强度因子的累计分布函数,基于两参数泊松分布建立了与最大应力强度因子有关的竞争失效模型。通过模型计算结果,可以得出在任一最大应力强度因子下试样发生各种失效模式的概率,且经分析对比,本文中TC4两种疲劳失效模式的失效概率评估结果与试验数据吻合较好,为分析TC4钛合金超高周疲劳状态下的疲劳失效模式提出了新的评估方法。     

Determination of normal and shear residual stresses from fracture surface mismatch

The contour method of residual stress measurement has recently been adapted to measure fractured, rather than cut specimens. The fracture contour method was capable of determining normal residual stresses acting prior to the plane-strain failure of a large aluminium alloy forging, but shear residual stresses could not be measured (Prime et al., 2014).We demonstrate that the application of digital image correlation to topographic measurements of a fracture surface pair allows the determination of shear residual stresses in addition to the normal stress component. Miniature compact tension samples were extracted at an angle from a bent beam to give a known variation in normal and shear residual stress on the fracture plane. The material used was a metal matrix composite, which could be deformed plastically to introduce a known distribution of stresses and also present limited plasticity upon fracture, allowing plane-strain condition in a small specimen. The samples were fractured at cryogenic temperatures to further restrict plasticity. Although the fracture surface was non-planar and evidence suggested the occurrence of plasticity near the edges, experimental results correlated fairly well with the calculated normal and shear residual stress profiles.     

Constant stress intensity factors through closed-loop control

A new way of obtaining a constant stress intensity factor is achieved for any test specimen geometry subjected to closed-loop control loading. In contrast to using only load or displacement control the method draws on combined feedback from both displacement and load sensing, reducing the variation in the stress intensity factor by two decades compared to that if tested under constant load or displacement. A change in the signal mix ratio for a rectangular compact tension specimen is equivalent to changing the angle of a tapered compact tension specimen. This method can eliminate the need for the use of ‘complex’ geometries or for geometries in which measured crack lengths are used in conjunction with a computer or some other means to adjust the loading continuously for achieving a constant stress intensity factor.     

Residual stress effects in sharp contact cracking

A study is made of residual stress effects in the mechanics of median fracture in sharp indenter contact. Starting with a simplistic treatment of the elastic-plastic indentation field, the problem is conveniently resolved into two separable parts, involving reversible (elastic) and irreversible (residual) components. The assumption of geometrical similarity in the residual field about the deformation zone, later backed up by stress birefringence measurements, leads to a stress intensity factor for median crack propagation containing the elastic and residual parts as the sum of two terms. The resulting formulation for equilibrium fracture shows some differences in the crack response during the loading and unloading half-cycles. By imposing certain stress states on the specimen surface during indentation the residual component of the field may actually cause the median crack to continue in downward extension as the indenter is withdrawn, a response which is especially amenable to experimental investigation. Direct observations of median crack evolution in soda-lime glass confirm this and other essential predictions of the fracture mechanics theory. The contribution of the residual component to the crack growth is found to be by no means secondary in importance to that of the elastic component.     

Characterization of mixed-mode fracture based on a complementary analysis by means of full-field optical and finite element approaches

This paper focuses on the characterization of mixed-mode fracture parameters through use of two formalisms based on Crack Relative Displacement Factors and Stress Intensity Factors, respectively. The evaluation of Crack Relative Displacement Factors is based on a kinematic approach that integrates the experimental displacement field measured by a digital image correlation method. In parallel with this step, the stress intensity factor is calculated from a finite element analysis. The coupling between these two approaches allows for the identification of fracture parameters in terms of an energy release rate without any prior knowledge of material elastic properties. Depending on the mixed-mode configuration, the proportion of the energy release rate corresponding to opening and shear modes can be calculated. Moreover, the proposed formalism allows determining, in addition to fracture parameters, the local elastic properties in terms of reduced elastic compliance directly from the test sample. Experimental protocols are carried out using a Single-Edge notched specimen made from a rigid Polyvinyl Chloride polymer loaded at various mixed-mode ratio values.     

Investigation of the stress intensity factor changing on the hole crack subject to laser shock processing

Laser shock processing (LSP) is a new surface modification technology. The effect of the compressive residual stresses generated due to laser shock processing (LSP) on the stress intensity factor (SIF) of a through-the-thickness radial crack at the edge of the circular hole was investigated. The residual stresses around the hole induced by LSP were measured by using X-ray method. The relationship between the SIF and the residual stress was determined on the basis of the weight function theory in fracture mechanics. Crack propagation characteristics for such cracks subjected to the combination of the applied stress and residual stress were discussed. And the results showed that the compressive residual stress could lead to the decrement of the SIF. Moreover, the number of the laser shocks had an important influence on the SIF.     

A semi-theoretical and experimental approach for the determination of the stress intensity factors

The stress intensity factors for plexiglass plates containing edge cracks and subjected to either pure bending or tension are determined herein. The method of investigation was based on a semi-theoretical and experimental approach, where the stress intensity factors are expressed in terms of the measured diameter of the caustic, the crack length, and the width of the specimen. First, two basic crack arrangements (single and double edge cracks) were studied and then the method was utilized for the investigation of more complicated crack arrangements which are difficult or maybe impossible to be investigated otherwise. In particular, the stress intensity factor for plates having a sharp V-notch of various angles θ, and semi-infinite plates containing equal parallel edge cracks subjected to pure bending and tension respectively, were investigated in order to verify the validity of this method.     

Numerical investigations of the stepped double torsion fracture toughness specimen

This study aims at investigating the fracture behaviour of double torsion specimens using the finite element method. Typical double torsion tests encompass a series of constant-thickness specimens to evaluate the material plane strain fracture toughness. In contrast, the concept of using a novel variable thickness stepped specimen aims at deducing the fracture toughness using a single specimen. In this work, the feasibility of this approach is examined and the effect of the number of steps and fracture thickness in a specimen upon the resulting conditional stress intensity factor is evaluated. The finite element models employed experimentally determined values of the fracture load to evaluate the conditional stress intensity factor of the specimen. Finite element predictions were compared with earlier experimental results using both cast aluminium silicon alloy and gray cast iron specimens and good matching was observed between experimental results and numerical predictions.     

Anrißerzeugung in WC-Co-Hartmetall-Proben für die Messung des kritischen Spannungsintensitätsfaktors KIC

Precracking of WC-Co-Hardmetal-Specimens for Fracture-Toughness Testing The determination of a valid critical stress intensity factor K_IC requires an extremely sharp, well defined initial crack. Methods producing such a crack are well known for metallic materials, but they often can not be used with brittle materials, like cemented carbides or ceramics. Their low fracture toughness makes a controlled crack growth under pure tensile stress nearly impossible. More useful are precracking methods, utilizing a stress gradient to stop the crack at defined depth. A very simple methods uses the indentation of a hardness tester to produce a semi-elliptical surface crack, interfered with residual stresses. For different areas of application and specimen geometries, bridge indentation, wedge indentation and composite bending method produce cracks with a straight front. Also under cyclic loading, under tensile as well as under compressive stress, the creation of a sharp precrack, applicable in K_IC measurement, is possible.     

Significance of K-dominance zone size and nonsingular stress field in brittle fracture

Stress intensity factor has been used to characterize the fracture toughness of a brittle material. This practice is apparently based on the assumption that the singular stress alone at the crack tip is responsible for fracture and that the nonsingular part of the near tip stress has no effect on fracture. In this study, mode I fracture experiments were conducted on a brittle material (PMMA) with four different specimen configurations. The result indicated that fracture toughness cannot be described by stress intensity alone and that a second parameter representing the influence of the nonsingular stress is needed. A two-parameter fracture model was proposed and validated with the experimental result. This two-parameter model was shown to be able to account for various effects created by specimen configurations, crack sizes, and loading conditions, on the fracture behavior of brittle materials.     

The performance of isoparametric finite elements in stress intensity factor determination

Analysis of a compact compression specimen used for fracture toughness evaluation of cementitious materials is carried out by the finite element method using isoparametric elements. Both triangular and rectangular elements were used with those surrounding the crack tip being of the quarter point type. Solutions were obtained for different mesh subdivisions and convergenece curves for the stress intensity factor were obtained by several methods based on extrapolation and energy techniques. It is found that monotonic convergence was obtained for all cases considered. Employing uniformly graded rectangular element representations converged solutions for the stress intensity factor (assuming a 1 percent convergence criterion) were obtained by the energy methods using a total of 720 degrees of freedom for solving half the structure.Tests on modified 100 mm cubes with symmetrical notches were conducted to determine the fracture toughness. The fracture toughness was calculated from the stress intensity factor and the maximum load obtained from the tests which were conducted in a stiff Instron testing machine. The fracture toughness is found to be independent of the size of the notch.     

The effect of a residual stress field on fracture initiation in RPV steel

Whether flaws in structures containing residual (secondary) stresses will extend under particular operational (primary) loads depends on the extent to which the residual stress field affects: (a) the nature and distribution of initiators; (b) the combined (primary + secondary) stresses and strains experienced by potential initiators. This paper compares fractographic data from specimens loaded by only a primary stress with data from specimens also containing a tensile residual stress field. Three‐dimensional elastic–plastic finite element calculations are used to characterize the stress–strain conditions at the initiation sites at the onset of brittle fracture. The introduction of a residual stress changes the dominant stage in fracture nucleation from microcrack extension to particle cracking. This offsets some of the decrease in fracture toughness expected when the residual stress field increases specimen constraint.     

A method for determining dynamic stress intensity factors from cod measurement at the notch mouth in dynamic tear testing

A formula is derived for determining dynamic stress intensity factors directly from crack mouth opening displacements in dynamic tear test specimen. The results obtained by the present estimation method for stationary as well as propagating cracks agree excellently with those directly obtained through a highly accurate moving-singularity finite element method. The present method can also be applied for other types of specimen which have a relatively short edge crack without any loading on the crack surface. The present simple estimation method should be of great value in the experimental measurement of dynamic stress-intensity factors for propagating cracks in (opaque) structural steel dynamic tear test specimens.     

A STUDY OF RESIDUAL CAUSTICS GENERATED FROM FATIGUE CRACKS

Abstract— The method of caustics was used to determine the stress intensity factor of fatigue cracks in steel compact tension specimens. Under zero load a residual caustic was observed at the tip of a fatigue crack indicating the presence of a residual stress field. Caustics were generated at increasing static loads and the stress intensity factors were compared with those predicted by theory. It was found that the difference between each measured stress intensity factor and its corresponding theoretical value was a constant for the range of loads. This difference was shown statistically to be equal to the stress intensity factor determined from the residual caustic. The proposed mechanism for the formation of this residual caustic was probably due to crack tip plasticity effects and not due to crack closure. It was concluded that residual caustics can be measured to quantify crack tip behaviour in fatigue cracks and have been shown to be a useful tool in the measurement of residual stress fields.